The present invention relates to a printed circuit board and, more particularly, to a printed circuit board in which a capacitor is three-dimensionally formed in a substrate.
Conventionally, various types of so-called printed capacitors which are three-dimensionally formed in printed circuit boards have been proposed.
FIGS. 1A and 1B show an example wherein a printed capacitor is formed in a two-layered circuit board 2. A conductive film (first electrode portion) 5 is formed on one surface 1a of a board 1, and a conductive film (second electrode portion) 4 is also formed on a rear surface 1b. These conductive films 5 and 4 are arranged to oppose each other to sandwich an insulating substrate 2, thereby forming a printed capacitor C. Note that reference numeral 6 in FIGS. 1A and 1B denotes a through-hole; and 7, a through-hole terminal for electrically connecting the through-hole 6 to the surface 1a of the board 1.
The prior art shown in FIGS. 1A and 1B exemplifies a two-layered board. Furthermore, in order to facilitate an assembling process of parts, and the like, and to achieve high-density packaging, a method of integrally forming a capacitor part in a multi-layered printed circuit board has also been proposed.
FIGS. 2 to 4 show a conventional printed capacitor formed in a multi-layered printed circuit board constituted by three or more layers. FIG. 2 is a plan view of the printed capacitor, FIG. 3 is a sectional view taken along a line XXVI--XXVI in FIG. 2, and FIG. 4 is an exploded perspective view of the printed capacitor in units of layers. More specifically, a multi-layered printed circuit board 1 is constituted by alternately stacking two or more insulating layers 2 (three layers in FIGS. 2 to 4), and three or more conductive layers 3 (four layers in FIGS. 2 to 4). A pair of electrode films 4 and 5, which oppose each other to sandwich one insulating layer 2 therebetween are respectively formed on the two conductive layers 3. The pair of electrode films 4 and 5 serve as electrodes of a capacitor. In the example shown in FIGS. 2 to 4, the electrode film 4 on the inner layer side of the multi-layered printed circuit board 1 is connected to the uppermost layer of the multi-layered printed circuit board 1 via a plated through-hole 6.
In the printed capacitor shown in FIG. 1, the capacitance of the capacitor is assured by the areas of the conductive films 5 and 4 formed on the surfaces 1a and 1b of the board. When a large-capacitance capacitor is to be realized, the area of the capacitor on the board becomes large, and a compact structure cannot be realized.
Similarly, in the printed capacitor of three or more layers shown in FIGS. 2 to 4, the electrode films 4 and 5 must be formed on wide ranges of the conductive layers 3 so as to assure a prescribed electrostatic capacitance. For this reason, the ratio of the areas of the electrode films 4 and 5 to the conductive layers 3 of the multi-layered printed circuit board 1 becomes large. As a result, the packing efficiency of parts cannot be improved very much.